The determination of the presence or concentration of antigenic substances, for example, those associated with a wide variety of physiological disorders, in serum or other body fluids relies increasingly upon immunoassay techniques. These techniques are based upon formation of a complex between the antigenic substance being assayed and an antibody or antibodies in which one or the other member of the complex may be labelled, for example, by a radioactive element such as I.sup.125, which permits its detection and/or quantitative analysis after separation of the complexed labelled antigen or antibody from uncomplexed labelled antigen or antibody.
In the case of a competition immunoassay technique, the antigenic substance in a sample of fluid being tested for its presence competes with a known quantity of labelled antigen for a limited quantity of antibody binding sites. Thus, the amount of labelled antigen bound to the antibody is inversely proportional to the amount of antigen in the sample. By contrast, immunometric assays employ a labelled antibody. In such an assay, the amount of labelled antibody associated with the complex is directly proportional to the amount of antigenic substance in the fluid sample.
Immunometric assays have been found to be particularly well suited for the detection of polyvalent antigens, i.e., antigenic substances that are able to complex with two or more antibodies at the same time. Such assays employ a quantity of unlabelled antibody bound to a solid support that is insoluble in the fluid being tested and a quantity of soluble antibody bearing a label such as a radioactive isotope that permits detection and/or a quantitative estimate of the amount of the ternary complex formed between solid phase antibody, antigen, and labelled antibody.
In immunometric assays known to the prior art, typically a "forward" assay, in which the antibody bound to the solid phase is first contacted with the sample being tested to extract the antigen from the sample by formation of a binary solid phase antibody:antigen complex, is employed. After a suitable incubation period, the solid support is washed to remove the residue of the fluid sample, including unreacted antigen if any, and then contacted with a solution containing a known quantity of labelled antibody.
After a second incubation period to permit the labelled antibody to complex with the antigen bound to the solid support through the unlabelled antibody, the solid support is washed a second time to remove the unreacted labelled antibody. In a simple "yes/no" assay to determine whether the antigen is present in the sample being tested, the washed solid support is tested to detect the presence of labelled antibody, for example, by measuring emitted radiation if the label is a radioactive element. The amount of labelled antibody detected is compared to that for a negative control sample known to be free of the antigen. Detection of labelled antibody in amounts substantially above the background levels indicated by the negative control is interpreted to indicate the presence of the suspect antigen. Quantitative determinations can be made by comparing the measure of labelled antibody with that obtained for standard samples containing known quantities of the antigen.
This kind of assay is frequently referred to as a "two-site" or "sandwich" assay since the antigen has two antibodies bonded to its surface at different locations. This and related techniques are described by Wide at pp. 199-206 of "Radiommunoassay Methods", Edited by Kirkham and Hunter, E. & S. Livingstone, Edinburgh, 1970. An assay based on this technique for the detection of the antigen associated with serum hepatitis using an 125.sub.I labelled antibody is described in U.S. Pat. No. 3,867,517.
Despite their great utility, the prior art immunometric assays have been recognized to be slow procedures, in part because two washing steps are required and because lengthy incubation periods are required to reach equilibrium, i.e., the point at which the amount of complex formed does not change with increasing time.
To eliminate at least one of the washing steps associated with this procedure, so-called "simultaneous" and "reverse" assays have been proposed. A simultaneous assay involves a single incubation step as the antibody bound to the solid support and the labelled antibody are both added to the sample being tested at the same time. After the incubation is completed, the solid support is washed to remove the residue of fluid sample and uncomplexed labelled antibody. The presence of labelled antibody associated with the solid support is then determined as it would be in a conventional "forward" sandwich assay.
A reverse assay involves the stepwise addition first of a solution of labelled antibody to the fluid sample followed by the addition of unlabelled antibody bound to a solid support after a suitable incubation period. After a second incubation, the solid phase is washed in conventional fashion to free it of the residue of the sample being tested and the solution of unreacted labelled antibody. The determination of labelled antibody associated with the solid support is then determined as in the simultaneous and forward assays.
Both the simultaneous and reverse assay techniques require a sufficient excess amount of solid phase antibody to bind most or all of the antigen present to avoid a high dose hook effect where artificially negative or low quantitation of antigen is observed at extremely high concentrations of antigen. For this reason, the forward assay has been the approach preferred by the prior art. That is because large amounts of highly purified, active antibody specific to the antigen of interest for preparing a solid phase with sufficient antigen binding capacity is difficult to obtain from the "polyclonal" antibodies used in prior art processes. Methods for affinity purifying such antibodies have generally been time consuming and resulted in low yields and loss of high affinity antibodies. When an immunogenic substance is introduced into a living body, the body's immune system reacts by generating antibodies to every site on the immunogen it recognizes. A large immunogenic protein molecule may have dozens of sites and a foreign cell may have hundreds. Thus, while each antibody producing cell produces antibody specific for a single antigenic site the immune system has generated a specie of specific antibody producing cells for each immunogenic site recognized. In addition, the body has produced relatively large quantities of antibodies to antigens other than the one of interest such that most of the antibody in the polyclonal mixture is not specific for the antigen of interest. Accordingly, the antibodies used in prior immunometric assays are necessarily "polyclonal" in nature since the antibodies are derived from antisera raised in a conventional manner in animals and their purification is difficult.
When employing conventional polyclonal antibody mixtures in the reverse and simultaneous assays, the formation of a "sandwich" comprising the antigen complexed by two or more labelled antibodies which complex with the antigen at different sites is possible. These complexes could remain soluble in the sample being tested, be removed by subsequent washing steps, and not "counted" when the solid phase is analyzed for solid phase bound labelled antibody. If this happens to a significant extent, sensitivity of the assay is reduced and erroneous results may arise. However, if the unlabelled bound antibody is added to the sample first as in the forward sandwich assay, steric considerations prevent formation of a sandwich comprising the antigen complexed to two or more unlabelled antibodies where labelled antibody is excluded from also binding to the antigen. Accordingly, the antigen is free to react with a labelled antibody molecule. Nevertheless, it has been proposed to use a simultaneous assay for human thyroid stimulating hormone (HTSH) by employing a large excess of the unlabelled antibody bound to a solid phase to minimize formation by soluble labelled antibodies. See Jeong et al., "Comparison to Radioimmunoassay (RIA) with a Unique, Single-Incubation Two-Site Immunoradiometric Assay (IRMA) as Applied to the Determination of Human Thyroid Stimulating Hormone (HTSH)", Bio-Rad Laboratories, 1979.
It has also been proposed to use a reverse assay for HTSH, hepatitis associated antigen (HAA) and carcinoembryonic antigen (CEA) by employing a quantity of labelled antibody sufficient to assure a labelled antibody:antigen complex but insufficient to form a "sandwich" of all the antigen present in a sample. See U.S. Pat. No. 4,098,876.
Since all three of the procedures known to the prior art use a polyclonal mixture of antibodies, the potential for cross-reaction with other materials in serum or other fluid than the antigen for which the test is intended is increased. The occurrence of cross-reactivity with other antigens also reduces the sensitivity of the test for the suspect antigen and increases the prospect of a "false-positive" assay. Furthermore, the use of polyclonal antibodies in a simultaneous or reverse assay requires a careful consideration of the amount of labelled antibody used relative to the amount of solid phase antibody and/or antigen present.
In view of these shortcomings, the limitations to the immunometric procedures known to the prior art are readily apparent. The conventional forward assay is time consuming; the simultaneous and reverse assays are accomplished with fewer steps but require large quantities of solid-phase specific antibody and are not well suited to determination of small concentrations of antigen since formation of a sandwich of the antigen with a multiple number of labelled antibody molecules competes with formation of the sandwich comprising bound antibody:antigen:labelled antibody; and all are subject to misinterpretation of false-positives due to the polyclonal nature of the antibody.
Accordingly, one object of the present invention is to provide an improved process for the immunometric assay for antigenic substances.
More specifically, an object of the present invention is to provide more rapid immunometric assay techniques.
Another object of the present invention is to provide more sensitive immunometric assay techniques.
Yet another object of the present invention is to provide improved "simultaneous" and "reverse" immunometric assays.
The manner in which these and other objects are realized by the present invention will be apparent from the summary and detailed description set forth below.